Originally Posted By: patuca
On one of my favourite trail rides it's common to see my garmin gps trip computer display a heavy (+20%) error in certain areas. Plotting the track on my mapping program (GPS Trackmaker) shows a more accurate speed but there is always some positive error.

Tracking this down I read an explanation something like this:

On the screen a GPS receiver displays the "EPE" or the "expected positional error" and even under perfect conditions this is error is sometimes 10 feet or even more for each point on a track.

That means the gps is tracking the motion not as a straight line, but as a series of jogs from point to point which makes the distance appear longer and thus the speed is instantaneously presented as a faster speed.

GPS receivers have a built in algorithm that averages the jogs when the track is saved or plotted, but just reading the active track data might not be accurate.

The error is minimized with good satellite geometry, clear view and many locked up satellites...and a straight course, but it's common for there to be a slight positive error.

I think you can minimize the problem by using the gps to set a measured mile and timing a run rather than noting the gps reading.



Very interesting. Common conception is that a GPS is a more accurate reference for speed readout. This dispels that theory.
My son has a TW200, and noticed a similar discrepancy in that bike too. Makes me wonder if Yamaha speedo's are set different than other jap bikes.

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